One of many challenges for manufacturer of machine parts that are subjected to intensive abrasive wear, especially in conditions where abrasion wear is combined with impact loads, is to ensure a satisfactory longevity of these particular machine parts. Usually additional considerations, such as the fixing technique of the liner and/or the maintenance facility, for example, are also to be taken to account.
Various technical solutions presently used in mining and similar industries to protect machine parts from wear are able to meet, to some degree, these requirements, by using materials that have good abrasive and/or impact resistance, design flexibility, and good weldability.
Austenitic steels with a 13% Mn by weight, for example, have very good toughness and strength and are used in extremely hard wear conditions, including impact wear conditions that occur for example in conical and jaw crushers, or in excavator teeth. However, these steels have a relatively low hardness (about 220 HB) and therefore a low abrasive resistance (see Metals Handbook, 10th edition, 1990, ASM International, Material Park, Ohio). Moreover, due to their poor weldability, they require special welding rods and higher welding time and general costs.
Hi-Cr cast irons, described for example in G. Laird, R. Gundlach, K. Rohrig. Abrasion-Resistant Cast Iron Handbook, AFS, Illinois, 2000, have very good hardness and abrasive wear resistance resulting from a microstructure comprising extremely hard chromium carbides dispersed in a martensite or martensite-austenite matrix. However, this increased hardness leads to a very low ductility and for this reason the use of these materials in impact intensive conditions is either counterproductive or limited. Moreover, these cast irons cannot be easily welded and, therefore, have to be fixed on the protected surface by bolting.
Another group of wear resistant materials comprises low carbon heat-treated steels like, for example, Hardox™, AR steel, Astralloy™. They have high strength, good toughness, and good hardness (up to 550 HB) while remaining weldable to a certain extent. As compared to ferrite and pearlite steels, they demonstrate an increased wear resistance, however, they are significantly inferior to Hi-Cr cast irons from a wear resistance point of view. Their microstructure lacks carbides or other phases comparable, from the hardness point of view, with the quartz, which is known as one of the widest spread wear causing components of all abrasive materials. Moreover, these steels can exclusively be used to cover flat surfaces, since they are produced by rolling methods.
There have been attempts to combine the properties of tough, ductile materials, such as steels, and highly wear resistant but brittle materials, like Hi-Cr cast irons, by laminating such materials together into one product. Brazed laminated plates consist of a massive Hi-Cr cast plate jointed with a mild steel under-plate by brazing. Such products combine the high wear resistance of Hi-Cr cast iron with the good weldability properties of mild steel. However the brittleness of the Hi-Cr cast iron reappears in the top part of this products and the brazed bond between two parts may fail. There are well reported cases where chunks of the Hi-Cr cast iron block were separated from the laminated plate, resulting in serious damages to the machinery down the production line and, consequently, significant downtimes. Moreover, brazed laminated plates cannot be manufactured to fit curved surfaces or to have a variable thickness.
Popular hard faced plates, such as provided by the company BROSPEC INC. for example, consist of mild steel flat bars covered by welding with alloys in which carbides are dispersed in a mainly austenitic matrix. These products have a good weldability but they inherit drawbacks from the automatic welding process used for their manufacturing. First, they may only be placed on flat surfaces. Secondly, the total thickness, even in multilayer product, is very limited (usually ½″ up to ¾″) by metallurgical reasons. Third, the wear resistant layer has high internal stresses due to a number of factors including high thermal gradient, different thermal coefficients of the mild steel and the alloy itself as well as high cooling speed. These stresses eventually cause cracking of the hard faced layer with subsequent crumbling of the layer. After welding, although the austenitic microstructure is far from being optimal, there is no possibility to improve it by heat treatment because of those internal stresses and the divergence of the mechanical properties.
Another group of technical solutions to increase the wear resistance of the machinery includes placing hard inclusions made of Hi-Cr cast iron or tungsten carbides in selected parts of the machinery. For example, U.S. Pat. No. 5,439,751 describes an ore pellet cast grate cooler side plate having a bottom surface containing embedded insert made of Hi-Cr cast iron.
U.S. Pat. No. 5,081,774 and U.S. Pat. No. 5,066,546 describe composite casting of an excavator tooth in which the critical wear areas are protected by Hi-Cr cast iron inserts or other material.
U.S. Pat. No. 1,926,770 proposes to insert tungsten carbide items in grey cast iron products.
The aforementioned solutions prove inefficient in protecting sophisticated structures, concave or convex surfaces and super-thick parts or pieces having a variable thickness.